Coal Swelling Model for High Heating Rate Pyrolysis Applications

Abstract

Thermal swelling of coal during pyrolysis strongly influences combustion and gasification rates. Coal swelling is known to vary strongly with coal rank, heating rate, and total pressure. New experiments confirm and clarify previous observations that maximum swelling occurs for high-rank bituminous coals at heating rates slightly below 104 K/s. Advanced swelling models based on bubble physics yield good qualitative trends at low heating rates, but they fail to predict observed decreases in coal swelling as particle heating rates increase beyond 104 K/s. An empirical swelling correlation for computational fluid dynamics (CFD) applications that correctly describes experimentally observed trends with the heating rate is proposed. Model parameters were fit to data from the literature using a coal rank index based on the chemical structure of the coal. The correlation has a form that allows for calculation of shrinkage for lignites, increasing swelling ratios as rank increases from sub-bituminous to bituminous ranks, and decreasing swelling ratios as rank increases from low volatile bituminous to anthracite. The correlation accurately predicts swelling as a function of coal rank and heating rate at atmospheric pressure.